In such a display apparatus that includes a cathode-ray tube for example, an antireflection film is stuck or its display surface in order to realize a high picture quality by reducing reflections of surroundings, improving the contrast, and so on. The antireflection film is formed on a transparent substrate.
It is well known that the conventional antireflection film is formed of a laminate of silicon oxide and titanium oxide. FIG. 1 shows a conventional typical antireflection film. This antireflection film 1 includes an antireflection subfilm 7 of a four-layer structure formed on one side of a transparent substrate 2 made of a resinous substrate (filmy substance) of, e.g. polyethylene terephthalate (PET) or the like. The antireflection subfilm 7 is formed by laminating successively a first layer of high-refractive-Index titanium oxide (TiO2) film 3, a second layer of low-refractive-index silicon oxide (SiO2) film 4, a third layer of high-refractive-index titanium oxide (TiO2) film 5, and a fourth layer of low-refractive-index silicon oxide (SiO2) film 6. The thickness of the first layer of titanium oxide film 3 is 13 nm; the thickness of the second layer of silicon oxide film 4 is 20 nm; the thickness of the third layer of titanium oxide film 5 is 98 nm; the thickness of the fourth layer of silicon oxide film 6 is 92 nm. Optical characteristics of the antireflection subfilm 7 are such that the average reflectance Rave is 0.28% and the maximum reflectance P is 1.19% (both are values in the range of wavelengths from 450 nm to 650 nm).
The antireflection subfilm 7 can be formed by a film-forming apparatus such as a sputtering system using targets of metal oxide and metal nitride or a reactive sputtering system using metal targets.
When the film-forming apparatus by reactive sputtering is employed, if a speed of forming the silicon oxide film is 60 nm×m/min and a speed of forming the titanium oxide film is 12 nm×m/min, then the number of cathodes (so-called targets) required for obtaining a running speed of 0.5 m/min of the filmy transparent substrate will be 8 pairs. Because of a slow speed of forming the titanium oxide film, it is difficult to obtain the transparent film at a high film-forming speed. Thus, in order to increase the film thickness, the number of targets must be increased as follows:
Details of the number of cathodes are SiO2 (one cathode)/TiO2 (five cathodes)/SiO2 (one cathode)/TiO2 (one cathode).
Incidentally, as described above, although the titanium oxide film and silicon oxide film can be formed using the sputtering by targets of metal oxide and metal nitride or the reactive sputtering by metal targets, a speed of forming the transparent titanium oxide film having a high refractive index is so small between them that productivity of the antireflection subfilm is low. In addition, when forming a film of metal oxide and metal nitride, the reactive sputtering can raise the film-forming speed rather than the sputtering. Therefore, from an industrial viewpoint, an optical design and film structure is desired, which are capable of making a high-refractive-index layer of low film-forming speed as thin as possible.
By the way, a speed of forming a typical titanium oxide film is about 15% of that of a silicon oxide film in case of reactive sputtering of a titanium material. For example, when forming the titanium oxide film while the resinous film forming the substrate is kept running at a speed of 1 m/min, its thickness of 7˜12 nm can be obtained. On this occasion, a speed of forming the titanium oxide film is 7˜10 nm×m/min. On the other hand, the silicon oxide film used as a low-refractive-index layer can provide a film-forming speed of about 45×60 nm by the reactive sputtering. This means that productivity of the antireflection film depends on a thickness of titanium oxide film having a high refractive index and low film-forming speed.
The optical films composing the antireflection film are industrially produced by the above-described reactive sputtering process using metal targets. However, wish this reactive sputtering technique, if gas is gushed out of the substrate, the film-forming becomes unstable. For example, while an amount of produced oxygen when the titanium oxide film is formed is about 10 sccm, if an excessive activated gas is supplied by the gas gushed out of the substrate (so-called outgas), then the film-forming speed and optical constants will change, thus making it difficult to obtain a desired film thickness and optical characteristics. When the resinous substrate is employed for the transparent substrate of antireflection film, plenty of moisture may be spouted during the sputtering. If a residual gas in a film-forming chamber increases due to the gushed gas caused by that moisture, the film-forming speed goes down, making a bad effect on the film-forming.
On the other hand, when, for example, the antireflection subfilm in four layers of silicon oxide film and titanium oxide film is formed on the PET film substrate, the conventional antireflection film itself has a bad permeability against moisture. Consequently, when the antireflection film is stuck on the surface of a CPT panel, the antireflection subfilm or the antireflection film itself deteriorates, exfoliates and falls away under a high-temperature and high-humidity environment.